The present invention relates to an apparatus for detecting errors (hereinafter, simply referred to as line errors) in a communications line as accurately as possible and to improve the precision with which the quality of transmission is evaluated.
Communications networks have come to play an increasingly important role in social and economic activities. As there is now a wide demand for a broadband service, the reliability of a network has definitely become a factor of foremost importance. At present, an increasing number of dedicated lines carrying a DS3 signal (third-level signal in the digital hierarchy) are being provided to North American subscribers to the broadband service. Such a network is characterized by a direct interface with the subscriber, and so the quality of the line is especially important in the maintenance/operation of the network. Hence, it is required that the quality of the line be accurately evaluated.
FIG. 1 shows a location in the line carrying the DS3 signal, at which location the line quality is monitored. A DS1 signal is multiplexed by means of a multiplexing unit MUX and sent to an optical transmission line via a terminal office unit LTE. The quality of the line is monitored by means of the terminal office unit LTE.
As mentioned above, it is necessary for the carrier providing the service using the DS3 signal to monitor the quality of the line to which the subscribers are connected. A method for detecting errors in the code sequence produced in accordance with the B3ZS (Bipolar with 3 Zero Substitution) coding (a coding technique adopted for the DS3 signal) is proposed by Bellcore.
As shown in FIG. 2, according to the B3ZS coding, the code "00V" or "B0V" is substituted for a string of three zeros in ordinary AMI (Alternate Mark Inversion) bipolar signal bits is substituted for in accordance with a predetermined rule. Referring to FIG. 2, V indicates a violation pulse which interrupts the alternating pattern of a bipolar signal, B indicates the bipolar pulse, and + and - signify the polarity of a pulse. The predetermined rule requires that the substitution be performed such that an odd number of bipolar pulses B occur between one violation pulse V.sub.n and the next violation pulse V.sub.n+1.
In the example shown in FIG. 2, the first string of three zeros "000" in the data sequence "01000110001" is converted into "00V" and the next string of three zeros "000" is converted into "B0V" so that an odd number of (in this case, three) bipolar pulses intervene between the violation pulses V.sup.+ and V.sup.-. The result of this is the code sequence "0B.sup.+ 00V.sup.+ B.sup.- B.sup.+ B.sup.- 0V.sup.- B.sup.+ ".
By monitoring such a code sequence at the receiving end of the DS3 signal, it is possible to monitor the quality of the line. That is, when the occurrence of a violation pulse V which is not produced in accordance with the above-mentioned rule is detected, it is considered that there is an error in the code transmitted over the line, and the error is detected.
As mentioned before, it is possible to detect a line error by monitoring code sequences of the DS3 signal encoded in accordance with the B3ZS coding. In the error detection method as proposed by Bellcore, the line error is detected by identifying the BPV (Bipolar Violation). In this method, violation pulses V other than those occurring in the sequences "00V" or "BOV", which sequences are obtained as a result of converting the strings of three zeros in the DS3 code sequence, are designated as the "BPV", that is, the line error. The quality of the line is evaluated by counting the occurrences of BPVs.
A line error detection apparatus faithfully implementing the above-mentioned definition has the circuit configuration as shown in FIG. 3. Referring to FIG. 3, a B/U converter 20 converts bipolar pulses B in the DS3 signal input from the line into unipolar pulses. A B3ZS decoder 21 decodes the B3ZS code sequence output from the B/U converter 20 and delivers the decoded result to a multiplexing unit 22. An extraction part 23 extracts violation pulses V included in the patterns "BBOV" or "B00V" from the B3ZS code sequence output from the B/U converter 20. A BPV pulse detection part 24 detects a line error by identifying the BPV, the BPV being the violation pulses other than the extracted violation pulses. The BPV is designated as the error pulse and the occurrences thereof are counted by means of a counter 4. The count provided by the counter 4 signifies the quality of the line.
Tables 1 and 2 below list exemplary patterns of errors in code sequences. It will be noted that the conventional line error detection circuit can detect only 40% of the total number of occurrences error.
In Tables 1 and 2, B indicates a positive bipolar pulse B.sup.+, b indicates a negative bipolar pulse B.sup.-, V indicates a positive violation pulse V.sup.+, and v indicates a negative violation pulse V.sup.-. The mark "'" is attached to a bit where an error has occurred, and the mark ".sub.- " is attached to a bit where an error is detected. A sample data sequence of "1000000000100000100011" which is translated into "b00vB0Vb0vB00V00b00vBb" as a result of the B3ZS coding, is used in these table. In each row of Tables 1 and 2, it is assumed that an error has occurred in the bit position indicated by the mark "'", and that the original bit is transformed into the bit shown next to the mask "'". Table 1 shows cases where certain B, b, V and v pulses are transformed into a "0" pulse, or where a "0" pulse is transformed into a B, b, V or v pulse. Table 2 shows cases where a B pulse is transformed into a b pulse or vice versa, or where a V pulse is transformed into a v pulse or vice versa.
TABLE 1 ______________________________________ (cases where the transformation B, b, V, v .fwdarw. 0 or 0 .fwdarw. B, b, V, v occurs) Original Pattern No. of Case 1000000000100000100011 Detected No. b00vB0Vb0vB00V00b00vBb Errors Note ______________________________________ 1 b000B0V 1 0B0V 2 b00v00Bb0v 0 3-1 b00vBVVb0v 1 BV 3-2 b00vBbBb0v 0 4 b00vB00b0v 1 0b0v 5 b00vB0V00bB00V 0 6-1 b00vB0VbBbB00V 0 6-2 b00vB0VbvvB00V 1 bv 7 b00vB0Vb00B00V 0 8 b00vB0Vb0v000B00b00v 0 9-1 b00vB0Vb0vBV0V 1 BV 9-2 b00vB0Vb0vBb0B00b00v 0 10-1 b00vB0Vb0vB0VV00b00v 0 10-2 b00vB0Vb0vB0bB00b00v 0 11 b00vB0Vb0vB00000b00v 0 12-1 b00vB0Vb0vB00VV0b00v 0 12-2 b00vB0Vb0vB00Vb0v00v 0 ______________________________________
TABLE 2 ______________________________________ (cases where the transformation B .revreaction. b or V .revreaction. v occurs) Original Pattern No. of Case 1000000000100000100011 Detected No. b00vB0Vb0vB00V00b00vBb Errors Note ______________________________________ 1 b00BV0V 1 Bv 2 b00vv0Bb0v 0 3 b00vB0bv0v 1 bv 4 b00vB0VV0bB0V 0 5 b00vB0Vb0BV00V 1 BV 6 b00vB0Vb0vv00B00b00v 0 7 b00vB0Vb0vB00b00v00v 0 8 b00vB0Vb0vB00V00V00bBb 0 9 b00vB0Vb0vB00V00b00BVb 1 BV 10 b00vB0Vb0vB00V00b00vvv 0 11 b00vB0Vb0vB00V00b00vBV 1 BV ______________________________________
Referring to Tables 1 and 2, it will be noted that in those cases where the number of detected errors is 0, the 1-bit line error occurring in the bit position indicated by the mark "'" is not properly detected because the code sequence produced as a result of the error is seemingly normal.
Thus, it is inherently impossible to detect 100% of the errors by checking the code sequence. Below there are shown cases other than those listed in Tables 1 and 2 where a seemingly normal code sequence is produced as a result of errors occurring in two consecutive bits, with the result that the errors are not properly detected.
______________________________________ 1 "V.sup.- B.sup.+ B.sup.- B.sup.+ B.sup.- B.sup.+ 0V.sup.+ " .fwdarw. "V.sup.- B.sup.+ 0'0'B.sup.- B.sup.+ 0V.sup.+ " 2 "V.sup.- 00B.sup.+ 00B.sup.- B.sup.+ 0V.sup.+ " .fwdarw. "V.sup.- 000'000'B.sup.+ 0V.sup.+ " ______________________________________
where "'" indicates a bit where an error has occurred.
Thus, it is impossible, with the conventional definition, to detect an error including a string of zeros. Therefore, there is a need for developing an error detection technique adapted for detecting errors as accurately as possible except for those patterns that escape the detection, and for improving the accuracy in the line quality evaluation.